1. Field of the Invention
The present invention relates to an automatic pallet changer for use with a machine tool. More particularly, the present invention relates to a pallet transfer apparatus installed in an automatic pallet changer for use with a machine tool.
2. Description of the Related Art
Conventionally, various types of pallet transfer apparatus have been put to use, including:
(1) Those that use a hydraulic cylinder to directly transfer a pallet; PA1 (2) Those provided with a motor to transfer a pallet via a ball screw connected to the motor; and PA1 (3) Those that include an engaging member attached to a section of a chain to transfer a pallet.
All of the above-described apparatuses have a common drawback which arises from the fact that the stroke of the drive means is the same as the transfer distance of the pallet. If a pallet is to be transferred over a long distance, a long hydraulic cylinder, ball screw, or chain must be employed, making the entire apparatus undesirably large. In the case of an apparatus employing a hydraulic cylinder or a ball screw, abrupt acceleration and deceleration occur at the start-up and termination of the transfer operation, thus exposing the pallet and the work placed thereon to excessive shock or vibration. The type of apparatus using a chain has an advantage over the other two types in that the engaging member can be smoothly accelerated and decelerated around the sprockets on which the chain is fit. The problem presented by this type of drive means is that the pallet transfer distance is limited to the addition of the distance between the sprocket axes of the sprockets to the pitch circle diameter of the sprockets. Japan Examined Utility Model Application No. 62-4437 discloses a pallet transfer apparatus in which each chain is fixed to a base to transfer the sprockets twice the distance of the cylinder stroke while maintaining the sprocket pitch. The disadvantage of this apparatus is that the transfer distance is limited to only twice the cylinder stroke.